Glaucoma is a group of diseases that can damage the eye's optic nerve and result in vision loss and blindness. Glaucoma is a common problem in aging, where the pressure within the eye becomes elevated. There are three chambers of fluid in the eye: Anterior chamber (between cornea and iris), Posterior chamber (between iris, zonule fibers and lens) and the Vitreous chamber (between the lens and the retina). The first two chambers are filled with aqueous humor whereas the vitreous chamber is filled with a more viscous fluid, the vitreous humor. Normally aqueous humor secreted by the ciliary body courses through the posterior chamber, around the iris into the anterior chamber, and exits out through a spongy tissue at the front of the eye called the trabecular meshwork into Schlemen's canal, aqueous and episcleral veins. The delicate balance between the production and drainage of aqueous determines the eye's intraocular pressure (IOP). In patients with glaucoma, the outflow of aqueous fluid becomes restricted, thereby increasing the intraocular pressure, which in turn presses on the optic disk, and eventually compressing the incoming retinal blood vessels and the axons of the ganglion cells so that these vital cells die. Treatment to reduce the intraocular pressure is essential in glaucoma
Prescribing eye drops is usually the first step in lowering the IOP. Most medications reduce the eye pressure by decreasing the production of aqueous. However, because the aqueous supplies nourishment to the front of the eye, the body has a built-in mechanism that limits the extent the eye's pressure can be lowered with medication. Other medications work by improving the outflow of fluid from the eye. Medication treatment is expensive and often have side effects associated with it. Furthermore, these expensive drugs are not readily available in rural and undeveloped regions.
When medications no longer controls the eye pressure adequately, surgery may be needed. Trabeculectomy is one type of glaucoma surgery that is commonly performed. It involves making a tiny filtering valve in the sclera (the white of the eye). The aqueous drains out of the front of the eye through the tiny opening where it collects underneath the clear conjunctival tissue that covers the sclera. One of the most common problems with trabeculectomy is that it lowers the pressure too well, which can cause complications and significantly lengthen the recovery time. Another disadvantage of this procedure is that the body's natural healing process may gradually close the filter, causing the pressure to become elevated again. Another surgical procedure performed is iridectomy when iris root blocks the entrance to the trabecular meshwork thereby impeding the flow of aqueous humor. A hole in the iris (Iridectomy) can be surgically created to relieve the blockage. The disadvantage associated with this procedure is that the tissue of the eye may gradually close the opening created, thereby requiring the need for further treatment.
Drainage implant is another widely used corrective method for glaucoma treatment. One such device is discussed by Anthoney C. B. Molteno in U.S. Pat. No. 4,457,757. This device comprises of a rigid plate having a tube that extends into the intraoccular chamber. The aqueous humor from the intraoccular chamber flows onto the surface of the plate and is absorbed by the body. This implant is not desirable since plates are formed of rigid plastic material, which may cause post operative irritation and difficulty during insertion in the eye. Furthermore, the Molteno plate does not have any pressure controlling mechanism, and thus always has associated risk of causing hypotony.
U.S. Pat. No. 4,554,918, issued to Thomas C. White suggests one type of glaucoma valve where the aqueous humor flows from the intraoccular chamber through a tube into an external reservoir. The end of the tube in contact with the reservoir has unidirectional valve, which is basically a small opening. The small opening provides limited resistance to flow of the aqueous humor. When the reservoir gets filled, patient is supposed to press the reservoir so as to force aqueous humor out into the body for absorption. This feature makes this device undesirable for long term glaucoma treatment. Furthermore, the small opening can get clogged by biological particulates present in the viscous aqueous humor, posing potential risk of valve dysfuntionoing.
In a glaucoma shunt disclosed by Ahmed in U.S. Pat. No. 5,411,473, a valve is added to the basic Molteno design. The valve claims to provide some protection against excessive aqueous outflow during the first few days after surgery. The valve includes a thin Silicone elastomeric sheet sandwiched between two rigid plates and a tube is attached to the silicone sheet. One of the drawbacks in the design is that, the tube end in communication with silicone sheet, gets compressed when sheet is sandwiched, thereby closing the aqueous outlet. This feature can cause valve clogging from prolonged use. Furthermore, the plates are formed of a rigid plastic material, which makes insertion beneath the eye tissue difficult and time consuming. The rigid plastic material poses a risk of irritation and/or damage to adjacent vasculature and tissue. Other non-valved implant described in Baerveldt, U.S. Pat. No. 5,476,445, is basically a device for releasing Intraoccular pressure. Since, no pressure regulating mechanism is available, it could lead to hypotony. Furthermore, the implant employs temporary sutures, which need to be removed after period of time, which in turn requires another surgery.